This invention relates generally to digital communication over a band-limited channel and, more particularly, to techniques for compensating for signal attenuation in band-limited communication channels. There is a growing need to transmit high speed data over inexpensive communication media. For example, high-performance local area networks (LANs) typically use optical fiber or coaxial cable as a communication medium, but there is a need for a network design using a lower cost communication medium, such as unshielded twisted pairs of conductors, but having an equivalent performance to a network using a more expensive medium.
As described in the cross-referenced application, the use of unshielded twisted pairs of conductors in a token ring network presents a number of difficulties if transmission rates up to 125 Mb/s (megabytes per second) are to be attained. Copper conductors have a limited bandwith compared with more expensive communication media. Higher frequencies are significantly attenuated, and some form of compensation, usually referred to as equalization, is required. Equalization may be performed either before transmission, by boosting the amplitudes of higher frequency components of the signal to be transmitted, or after transmission, by amplifying the higher received frequencies. In either case, an equalizer is a relatively expensive component.
One common form of equalization circuit is a transversal filter implemented as a shift register. Digital samples of a signal are shifted through the register, each stage of which provides an output that can be multiplied by a controllable gain factor and fed to a summing circuit from which an output signal is derived. A clock signal shifts the signal samples through the register, at a higher rate than the rate at which the data changes state. Each output pulse from the equalizer is composed of a number of input pulses shifted in time and summed with different weightings to create a desired output signal shape.
The present invention provides a simple and much less costly alternative to this type of equalization circuit. An additional advantage of the invention is that it can be used to perform signal encoding simultaneously with its signal shaping function.